This invention relates to an improved process for the production of concentrated sulfuric acid from moist, SO.sub.2 -containing gases, formed in a furnace by the combustion of H.sub.2 S-containing gases or by the thermal cracking of aqueous waste sulfuric acid, wherein the SO.sub.2 -containing gases are cooled to a temperature within the range of from 400.degree. to 480.degree. C. The SO.sub.2 contained in the gases is oxidized to SO.sub.3 in a plurality of contact stages while lowering the temperature of the gases between the stages and absorbing the sulfur trioxide and water vapor, thereby forming sulfuric acid.
Moist, SO.sub.2 -containing gases are obtained during the combustion of H.sub.2 S-containing gases or during the thermal cracking of aqueous spent sulfuric acid. It is known to initially dry these moist, SO.sub.2 -containing gases and then to convert them catalytically to sulfuric acid. Drying of the hot combustion or cracking gases presupposes cooling and subsequent re-heating of the gases to the contact temperature.
It is also known to subject the moist gases to contact oxidation without previous drying and to form sulfuric acid by condensation of the SO.sub.3 and water vapor-containing contact gas. However, with this arrangement, there is obtained aqueous sulfuric acid, generally containing 78 to 80% by weight of H.sub.2 SO.sub.4, which corresponds to the moisture content of the gas. The possibilities for utilization of this aqueous sulfuric acid are limited. Additionally, it is also known to effect acid formation in the condensation stage at temperatures which are so high that the water vapor partially escapes out of the condensation stage with the waste gas, whereby a highly concentrated acid is produced. This mode of operation causes considerable material and operational problems due to the circulation of hot sulfuric acid in the condensation stage.
The underlying problem which this invention solves is the processing of moist, sulfur dioxide-containing gases, which are available at high temperature (such as for example H.sub.2 S-combustion gases or spent sulfuric acid cracking gases), into concentrated sulfuric acid having more than 95% by weight of H.sub.2 SO.sub.4, without incurring the result that, due to previous drying, the gas is heated due to indirect heat exchange and/or the acid condensation effected after contact oxidation is conducted at increased temperature, i.e., generally above 90.degree. C., whereby corrosion problems can arise.